Some secondary battery systems are provided with an abnormal voltage detecting apparatus that detects abnormal battery voltage, in order to prevent overcharging (i.e., an excessive increase in battery voltage) and overdischarging (i.e., an excessive decrease in battery voltage) of a secondary battery. For example, the voltage of the secondary battery is detected and if the detected voltage value is outside of a normal voltage range, the voltage in the secondary battery is determined to be abnormal. More specifically, upper and lower limit values for battery voltage are set in advance, and if during charging the detected voltage value exceeds the upper limit value, it is determined that the secondary battery is overcharged. Conversely, if during discharging the detected voltage value is below the lower limit value, it is determined that the secondary battery is overdischarged. Controlling charging and discharging of the secondary battery based on the determination by the abnormal voltage detecting apparatus enables overcharging and overdischarging of the secondary battery to be avoided.
However, if there is a malfunction in the abnormal voltage detecting apparatus such that it is no longer able to accurately determine whether the voltage of the secondary battery is within the normal voltage range, then it is no longer possible to avoid overcharging and overdischarging the secondary battery. Japanese Patent Application Publication No. 2004-312835 (JP-A-2004-312835) and Japanese Patent Application Publication No. 2007-20336 (JP-A-2007-20336) describe methods of diagnosing a malfunction in this kind of abnormal voltage detecting apparatus.
The method described in JP-A-2004-312835 is as described below. A secondary battery is charged using power generated by a regenerative operation of a motor mounted in a hybrid vehicle until the voltage of the secondary battery exceeds an upper limit voltage value. If an abnormality detection signal is output from the abnormal voltage detecting apparatus after the voltage of the secondary battery exceeds the upper limit voltage value, it is determined that the abnormal voltage detecting apparatus is operating normally, i.e., is not malfunctioning.
JP-A-2007-20336 describes a method of diagnosing a malfunction in an abnormality detecting apparatus using the voltage of a secondary battery while the secondary battery is discharging. More specifically, if an overcharging abnormality signal is output from an abnormality detecting circuit while the secondary battery is discharging, it is determined that there is a malfunction in the abnormality detecting circuit. Furthermore, if an overdischarging abnormality signal is output from the abnormality detecting circuit while the secondary battery is charging, it is determined that there is a malfunction in the abnormality detecting circuit.
However, with the diagnostic method described in JP-A-2004-312835, the secondary battery must continue to be charged using power generated by regenerative operation of the motor mounted in the hybrid vehicle until the voltage of the secondary battery exceeds the upper limit voltage value. Therefore, in order to determine that there is a malfunction in the abnormality detecting circuit in a hybrid vehicle that is running, the brake must continue to be applied until the voltage of the secondary battery exceeds the upper limit voltage value, which is not very practical. Moreover, charging the secondary battery until its voltage exceeds the upper limit voltage value overcharges the secondary battery, which may cause the secondary battery to deteriorate.
Also, the diagnostic method described in JP-A-2007-20336 is not able to detect some malfunctions. More specifically, for example, if there is a malfunction in which the actual voltage value of the secondary battery does not match the voltage value obtained by the abnormality detecting circuit, an overcharging abnormality signal (or an overdischarging abnormality signal) will not be output from the abnormality detecting circuit while the secondary battery is discharging (or charging). Therefore, this kind of malfunction is unable to be detected with the diagnostic method described in JP-A-2007-20336.
Also, both of the diagnostic methods described in JP-A-2004-312835 and JP-A-2007-20336 diagnose whether there is a malfunction in the abnormal voltage detecting apparatus using the battery voltage when the secondary battery, which is provided as a power supply in a hybrid vehicle or an electric vehicle, is charging or discharging. However, the battery voltage when the secondary battery mounted in a hybrid vehicle or the like is charging or discharging is unstable, so it may not be possible to accurately diagnose whether there is a malfunction in the abnormal voltage detecting apparatus.